The present invention relates to a high-voltage discharge lamp lighting device equipped with means adapted to adjust a peak value of a high-pressure pulse voltage at the time of start. The present invention further relates to a lighting fixture using the device.
A high-voltage discharge lamp is compact, yet can produce powerful beams of light, is near a point light source and is easily controlled in light distribution. For these reasons, the high-voltage discharge lamp has recently become a favorable alternative to incandescent and halogen lamps. A high-pressure pulse voltage of a few kV is generally required to start the high-voltage discharge lamp.
FIG. 14 shows a typical example of circuit structure for such a lamp. Reference character E denotes a DC power source, reference numeral 4 denotes a step-down chopper circuit, reference numeral 6 denotes a polarity reversal circuit, reference numeral 7 denotes a starting pulse generation circuit and reference character T1 is a high-voltage pulse transformer. The step-down chopper circuit 4 is formed of a switching element Q2, a diode D2, an inductor L3 and a smoothing capacitor C4. The configuration and operation of these components are generally known in the art and further description thereof is omitted. The polarity reversal circuit 6 is formed of a full-bridge circuit including switching elements Q3 to Q6 and applies a rectangular wave having a frequency of from a few dozen to a few hundred Hz to the starting pulse generation circuit 7 and the high-voltage discharge lamp 8.
The starting pulse generation circuit 7 has the transformer T1 including a secondary winding N2 serially connected to the high-voltage discharge lamp 8, a capacitor C1 for passing a pulse current to a primary winding N1, a charge resistor Rc and a switching element Q7 for discharging the capacitor C1.
The pulse generation operation is well known. For example, when the switching elements Q3 and Q6 of the polarity reversal circuit 6 are turned on and the switching elements Q4 and Q5 are turned off, a charge current flows from the capacitor C4 to the capacitor C1 via the switching element Q3, the primary winding N1 of the transformer T1, the charge resistor Rc, the capacitor C1 and the switching element Q6 to charge the capacitor C1 so that the switching element Q7 becomes positive. After that, when the switching elements Q3 and Q6 of the polarity reversal circuit 6 are turned off and the switching elements Q4 and Q5 are turned on, a high voltage obtained by adding the voltage of the capacitor C4 to the voltage of the capacitor C1 is applied to the switching element Q7 and exceeds a breakover voltage of the switching element Q7. As a result, an electric charge of the capacitor C1 is steeply discharged via the switching element Q7 and the steep discharge current flows into the primary winding N1 of the high-voltage pulse transformer T1, generating a pulse voltage. A high-pressure pulse voltage obtained by boosting the pulse voltage by the transformer T1 occurs in the secondary winding N2 to cause dielectric breakdown of the high-voltage discharge lamp 8. Generally, in the high-voltage discharge lamp 8, the starting pulse voltage is defined as 3-5 kV.
In the high-voltage discharge lamp lighting device, when the output wiring length (lamp terminal wire length) is increased, output capacitance increases and the starting pulse voltage is attenuated. This causes a problem that the starting pulse voltage of the lamp falls below a defined value and thus the lamp cannot be started. To prevent this problem. the high-voltage discharge lamp lighting device needs to be able to output 3-5 kV even when the output wiring length is relatively long. In this case, however, when the output wiring length is short, the pulse voltage becomes 5 kV or more, resulting in a possibility of leakage in the wiring or socket.
Japanese Unexamined Patent Publication No. 2007-52977 describes a previous attempt to address this problem. FIG. 15 shows the circuit structure of this particular example. A starting pulse generation circuit operates at the start-up of the high-voltage discharge lamp 8 and generates a high-pressure pulse voltage. The starting pulse generation circuit has a transformer T1, a switching element Q7 which can be turned on/off according to an external control signal, a capacitor C1 and an inductor L1 for over-current protection of the switching element Q7. This document proposes that a tertiary winding N3 of the transformer T1, a voltage divider circuit 11 and a pulse detection circuit 12 detect a peak value of the high-pressure pulse voltage and feed back the detection value so that a control circuit 9 may maintain the starting high voltage within a predetermined value.
However, the starting high voltage must be lowered to a voltage desired for feedback and a circuit structure such as a voltage divider circuit is required to lower the starting high voltage, leading to an increase in size and costs of the lighting device. This method is disadvantageous in part because the peak value of the starting high voltage cannot be accurately detected due to error factors such as an inherent variation in components of the voltage divider circuit and temperature characteristics. Further, since the peak value of the starting high voltage varies in generation timing, it is difficult to detect the accurate peak value depending on detection timing.